At least 36 people have been killed in a railway station stampede during the world's largest religious festival, India's Kumbh Mela.

An estimated 30 million Hindu pilgrims are bathing at the Sangam, the place near Allahabad where three rivers - the Ganges, the Yamuna and the mythical Saraswati - come together.

The stampede was triggered after an overcrowded railway station footbridge buckled and a railing collapsed, sending some people slipping down the stairs, a top state government official told Reuters, not wishing to be quoted by name.

The chief medical officer of Allahabad district said the toll rose to 36 after 14 more people died during treatment, Press Trust of India and NDTV news channel said.

Ash-smeared naked sadhu holy men led the ritual bathing before dawn, which is said to cleanse pilgrims of their sins, with millions following them into the swirling river waters at the festival site in Allahabad.

The population of the city swelled from 1.2 million to about 40 million on Sunday morning, with about 20 million packed inside the vast sealed off bathing area on the banks of the river.

Amid the crush police and thousands of volunteers on duty urged pilgrims to take one short dip and then leave the freezing waters to make space for the flow of humanity behind them.

In the two months from the start of the festival in January, officials believe as many as 100 million people will have passed through a temporary city that covers an area larger than Athens on a wide sandy river bank.

Hindu holy men and pilgrims bathe in the sacred Ganges to wash away lifetimes of sins.

Sunday was believed to be the most auspicious day of the festival.

'Out of the world experience'

The Maha Kumbh Mela, which began last month and ends in March, takes place every 12 years in Allahabad.

Smaller, similar events are held every three years in other locations around India.

Devotees believe entering the mighty rivers cleanse them of sin and free them from the cycle of rebirth.

Assorted dreadlocked holy men, seers and self-proclaimed saints from all over the country have assembled for the spectacle that offers a rare glimpse of the dizzying range of Indian spirituality.

Despite the hardships of waking early, plunging into the freezing and heavily polluted water and the crush of the crowds, pilgrims described being spiritually uplifted and amazed by the scale of the event.

The festival had its origins in Hindu mythology, which described how a few drops of the nectar of immortality fell on the four places that host the festival - Allahabad, Nasik, Ujjain and Haridwar.

Most devotees dunk their heads under the water, while some drink it and others bottle it and take it home as gifts.

Swapna Bhatia, an interior designer from New Delhi, called it "simply an out of the world experience".

"I feel so light now," Mr Bhatia said.

London man Malti Devi, 65, who was taking part in the festivities for the first time said "one dip in the river has the power to change life forever".